Steady froth formation is an undesirable phenomenon occuring in aerating media containing organic matter. Stability of froth is associated with the composition of complex media or products of metabolism of microorganisms which are not identified in many instances, and the fight against froth formation is frequently conducted on an empirical basis.
Negative effect of froth formation consists in the following:
a culture with froth may be let out of the fermenter through outlet ports;
the value of KL.sub.a (efficiency of oxygen absorption) decreases;
in chemostatic cultures (cultures with complete stirring to which a culture medium is added at a constant rate and from which a culture is withdrawn at the same rate while retaining the total volume unchanged) the gas content varies, hence the liquid volume of the culture medium becomes uncontrollable.
There is known an interface controller between two liquid media in a vessel used in a process of microbiological synthesis for suppressing froth in a fermenter by surface-active substances (froth suppressors) as exemplified in USSR Inventor's Certificate No. 485,423, [IPC C 05 d 9/12].
The above controller comprises a throttle installed in the interior of the fermenter at a preselected froth level and connected by a pipe to a comparator to control the feed rate of the froth suppressor into the fermenter. Another pipe connecting the comparator with the fermenter accommodates a pump providing for closed circulation of a gas mixture. The controller also comprises a vessel containing the froth suppressor and a pipe having a means for controlling the rate of feed of the froth suppressor from the vessel to the fermenter when an error signal is received by the comparator.
The aforedescribed apparatus operates as follows.
When froth in the fermenter is short of coming into contact with a throttle port, the pump acts to force the gas mixture via the throttle and the comparator from the fermenter and return the mixture back to the fermenter, the comparator remaining unengaged.
Conversely, when the froth reaches the throttle port, it is pumped by the pump whereby resistance in the throttle tends to increase sharply to result in an error signal received by the comparator which issues a control signal to the means for controlling the rate of feed of the froth suppressor for more froth suppressor to be fed to the fermenter.
The froth suppressor which is conveyed to the fermenter acts to suppress the froth thereby relieving the throttle port of the froth to result in a reduced flow resistance therein, the comparator assuming its initial state. Therewith, the means for controlling the feed rate of the froth suppressor terminates the supply of the froth suppressor to the fermenter.
The above apparatus may be employed both in large-scale production fermenters and in laboratories.
Inherent in such apparatus are disadvantages residing in that the provision for gas circulation through the comparator constituting a focal point of microorganism decay upsets the aseptic conditions for carrying out a microbiological process. In addition, supply of the froth suppressor by samples into the fermenter tends to result in overconsumption of the froth suppressor, since froth formation during a fermentation process is normally not programmable. On the other hand, the froth suppressor in the vessel is susceptible to stratification due to lack of stirring whereby froth suppression in the fermenter may be affected.
There is further known an apparatus for chemical froth suppression in a fermenter comprising a vessel containing a chemical froth suppressor fed to the fermenter by a pneumatic pump along a pressure pipe adapted to connect the vessel containing the froth suppressor with the fermenter, a nozzle disposed inside the fermenter at a preselected level of froth therein and connected by the pressure pipe with the vessel containing the chemical froth suppressor, the nozzle serving to form a jet of the froth suppressor for this jet to be received by an inlet port of a chamber accommodated in the interior of the fermenter, the inlet port being disposed coaxially with an outlet opening of the nozzle, a wall of the chamber opposite to the inlet port thereof being concaved to reflect the jet of froth suppressor escaping from the nozzle for the thus reflected froth suppressor to be sucked off and conveyed to the vessel containing the froth suppressor along a discharge pipe connecting this chamber with the froth suppressor containing vessel, and a throttle means serving to maintain a required pressure in the pressure pipe (cf. U.S. Pat. No. 4,302,545, Cl. [435-289].
In the above apparatus the pneumatic pump is arranged in the pressure pipe, while the throttle and the nozzle are positioned in succession downstream of the chemical froth suppressor subsequent to the pneumatic pump in the pressure pipe. This arrangement provides for a gravity drain of the froth suppressor into the vessel containing the chemical froth suppressor along the discharge pipe.
Discharge of the froth suppressor by gravity from the chamber causes suction or underpressure resulting in air bubbles affecting the rate at which the froth suppressor is withdrawn from the chamber which in turn may cause overflow of the chamber with the chemical froth suppressor and accidental discharge thereof through the inlet port of the chamber back into the fermenter.
The gravity drain of the chemical froth suppressor also fails to provide a sufficiently vigorous stirring thereof which may result in stratification, clogging of the throttle and loss of control over froth suppression.
One more disadvantage is that the gravity discharge of the chemical froth suppressor limits structural and operational capabilities of the apparatus requiring a careful selection of the flow area and height of the discharge pipe depending on the viscosity of the chemical froth suppressor employed.
The use of the discharge pipe for two purposes, particularly for discharging the chemical froth suppressor and equalizing pressure in the fermenter and the vessel containing the chemical froth suppressor affects the reliability of the apparatus. For example, when pressure in the fermenter drops due to reduced air supply for aeration, this pressure tends to equalize subsequent to the overflow of air along the discharge pipe from the vessel containing the chemical froth suppressor into the fermenter which may result in an accidental discharge of the froth suppressor into the fermenter.
Further, the arrangement of the throttle in the pressure pipe downflow of the pneumatic pump produces an overpressure in the pressure pipe which makes pressure sealing of the apparatus more complicated, affects operation of the pneumatic pump and brings down its operational reliability.